(a) Field of the Invention
The present invention relates to an optical disk drive capable of higher-speed seeking and, more particularly, to an optical disk drive capable of higher-speed seeking by controlling a fine actuator during a deceleration period of a rough actuator.
(b) Description of the Related Art
Optical disk drives for driving a compact disk (CD) or a compact disk read-only memory (CD-ROM) has an optical head irradiating an optical spot on a rotating disk file on which each data track includes data pits for read-out. The optical head detects the levels of intensity of a reflected light from the data pits, thereby reading data recorded on the disk file.
In CD drives or CD-ROM drives (hereinafter, simply referred to as CD drives), since the recording density of the optical disk file as viewed along a data track is constant over the whole track area in spite of radial position of the data track on the optical disk file, the linear speed of the disk file must be maintained constant by changing the rotational speed thereof depending on the radial position of the data track. The linear speed data for respective tracks are stored beforehand in the CD drives. The CD drives calculate the number of tracks to be crossed during a seeking operation based on the linear speed data stored beforehand, the address data of the present location (present address), on which the optical spot stays, and the address data of the target track (target address) included in the access command. The CD drives then move the optical spot to the target address based on the calculation.
FIG. 1 shows a seeking process effected by a conventional optical disk drive. After the optical disk drive receives an access command in step S43, the optical disk drive reads out the present address Tp in step S44. Subsequently, the optical disk drive first calculates radial position r.sub.1 of the present address Tp as viewed from the disk center, by using the following equation: ##EQU1## wherein r.sub.o, tp and V.sub.L are radial position of the minimum address from which the data is stored, the track pitch of the disk file and the linear speed at the present address, respectively. Thereafter, radial position r.sub.2 of the target address Tt also calculated by using the following equation: ##EQU2## wherein V.sub.L is the linear speed at the target track. Based on the results, the number Nt of the tracks (cross-track number Nt) disposed between the present address before seeking and the target address is obtained in step S45 by using the following equation: ##EQU3## The optical disk drive then moves the optical head based on the cross-track number Nt in a rough seeking operation, which is shown by steps S46 to S56. The rough seeking operation is effected based on a speed profile, such as shown in FIG. 2, including a first, acceleration period between t0 and t1, a second, constant speed period between t1 and t2 and a third, deceleration period between t2 and t3. In short, the rough seeking operation is controlled based on a so-called trapezoid speed profile.
During the acceleration period, the travelling speed of the optical head is raised up to a maximum speed Vmax, which is not higher than the critical speed below which the tracking error signal can be detected. After the maximum speed Vmax is obtained at time instant t1, the maximum speed Vmax is maintained, then deceleration period begins at the radial position where the number of the remaining tracks to be crossed hereinafter, i.e., remaining track number is at a threshold value, at time instant t2. The movement of the optical head is finished at the position where the remaining track number is zero at time instant t3.
In detail, after the remaining track number becomes a threshold value Nd, the speed V of the optical head for the deceleration period is calculated by using the cross-track number Nt and the count Nc of the crossed tracks (crossed-track count Nc) in step S51 as follows: ##EQU4## After the remaining track number Nt-Nc becomes zero at step S53, the rough actuator is stopped in step S55, then the tracking servo system is activated to read the present address data to finish the rough seeking operation in step S56.
In the conventional method, however, the location at which the rough seeking operation is finished often deviates from the target track due to errors in results of calculation of the linear speed or in the tracking error signal. Accordingly, read-out of the present address and a fine seeking operation follow the rough seeking operation. In detail, as shown in FIG. 3, one seeking operation includes the steps of receiving an access command (step S57), rough seeking (step S58), read-out of the present address (step S59), fine seeking (step S60), additional read-out of the present address (step S61) and awaiting disk rotation (step S63). The read-out can be executed by activating the tracking servo system during a stationary state of the optical spot. The fine seeking operation is effected to position the optical spot exactly at the target address and thereby requests the read-out of the present address before the fine seeking and a subsequent calculation of a second cross-track number in step S59.
In the conventional method, as described above, there is a limit for obtaining a higher-speed seeking to thereby reduce the access time for the optical spot because of a number of the sequential steps of rough seeking, read-out of present address, fine seeking etc.